With the ever increasing need to produce vehicles that are more fuel efficient, hybrid electric vehicles (HEV's) have provided an improvement in fuel economy over many conventional vehicles that utilize only an internal combustion engine to drive the vehicle. One of the primary advantages of an HEV is that it allows the vehicle to be powered by one or more electric motors under certain operating conditions. For example, if the speed of the vehicle is relatively moderate, and the battery or other electrical power source is sufficiently charged, the engine may be shut off, and the vehicle powered exclusively by the electric motors. As operating conditions change, the engine may be started to provide additional power, and if needed, operate a generator to charge the battery.
Various arrangements of internal combustion engines and electric machines—i.e., generators and motors—have been utilized in HEV's. For example, series arrangements, parallel arrangements, and power-split arrangements provide different ways for the engine and electric machines to interact to output torque to drive the vehicle. In one example of a power-split arrangement, an engine and an electric machine are both operatively connected to a planetary gear set, the output of which provides torque to vehicle drive wheels. One or more additional electric machines can be used to provide additional torque to the drive wheels, charge an energy storage device, such as a battery, provide electrical energy directly to another electric machine, or some combination thereof. In the power-split arrangement, power output from the engine flows through the planetary gear set, where a portion of the power is applied to the electric machine connected to the planetary gear set, and another portion of the engine power is transferred to the vehicle drive wheels.
In many examples of power-split arrangements, an engine output shaft is connected to one of the members of a planetary gear set, and an output shaft from an electric machine is connected to another of the members of the planetary gear set. These two torque-producing devices are often in-line with each other, such that their respective output shafts lie along the same axis. In some vehicle platforms, it may be desirable to reduce overall powertrain length by having torque-producing devices that are not in-line with each other.
In addition, it may be desirable to have a power-split configuration for an HEV powertrain that includes an electric machine having an output shaft that is not directly connected to a member of the planetary gear set. This would add flexibility to the configuration by providing a way to adjust gear ratios without changing the basic members of the transmission—e.g., the members of the planetary gear set.